The present invention relates to a fuel injection system for an internal combustion engine, especially to a fuel injection valve of a Diesel engine.
In a conventional fuel injection valve of the kind, a nozzle needle is reciprocally moved in a nozzle body having injection ports at the front end of the nozzle body, and thereby the injection ports are opened and closed in accordance with the reciprocal movement of the nozzle needle in order to intermittently inject the fuel. The injection valve is provided with a spring biasing the nozzle needle in a valve closing direction, while the fuel intermittently distributed from a fuel injection pump under high pressure biases the nozzle needle in a valve opening direction, thus the above reciprocal movement of the nozzle needle is performed.
It is recently required for a fuel injection system for an internal combustion engine to decrease a duration of fuel injection as much as possible in view of improving an operational performance of the engine and/or purifying the exhaust gases emitted from the engine. Especially, for the purpose of decreasing combustion noises of the engine, it is desirable for the injection valve to start the fuel injection not rapidly but gently.
In the above conventional fuel injection system, however, the pressure for the fuel distributed from the fuel injection pump to the respective injection valves is increased so as to decrease the duration of the fuel injection, inevitably resulting in a rapid starting of the fuel injection. Because, the nozzle needle reciprocally mounted in the nozzle body is biased toward the valve closing direction by the biasing spring alone. Accordingly, in the conventional fuel injection system, it is disadvantageous that the combustion noises are increased.